There are a number of commercially available products which provide sensing, control and communications in a network environment. These products range from elaborate systems having a large amount of intelligence to simple systems having little intelligence. By way of example, such a system may provide control between a light switch and a light. When the light switch is operated, a digital code is transmitted by one cell over power lines or free space or other media and is received by another cell at the light. When the code is received, it is interpreted and subsequently used to control the light. Such a system, comprising a network of intelligent cells in which the cells communicate, control and sense information, is described in U.S. Pat. No. 4,918,690, entitled "Network and Intelligent Cell for Providing Sensing, Bidirectional Communications and Control," which is assigned to the assignee of the present invention.
The transmitting and receiving of digital data is normally handled by a series of transceivers. Each of the transceivers is connected to an individual cell in a network. These transceivers may communicate with one another in numerous different ways over various media and at various baud rates. The transceivers may be connected to communications lines, such as an ordinary twisted pair or fiber optic cable. Other known communication media may be employed between the transceivers. One such medium is power distribution lines such as those found in virtually all homes, offices, and factories. In many cases, these power lines distribute 120 volt AC (alternating current) to wall sockets, thereby supplying power to various devices such as appliances, computers, lights, etc. Because power lines are designed primarily for transmitting power, they tend to be far from ideal for communication purposes.
One of the major problems in using power distribution lines for communications is the difficulty in dealing with numerous noise sources and significant signal attenuation which arise in the power line environment. A simplified view of a typical power line environment is shown in FIG. 1. As can be seen in FIG. 1, a house 49 is shown having a number of devices each coupled via a single communications medium which, in this case, is the standard electrical power line that exists in most houses. Power line communication (PLC) apparatus 60 is plugged into one receptacle of wall outlet 52 (alternatively, wall outlet 52 may incorporate the communications intelligence of PLC apparatus 60). Wall outlet 52 is connected by house wiring 53a to a main circuit breaker panel 50 of the dwelling. Other wall outlets, such as outlet 54, are also connected to circuit breaker panel 50 via house wiring 53b in parallel with wall outlet 52. Another PLC apparatus 62 is shown plugged into one receptacle of wall outlet 54 of FIG. 1. Household appliance 56 is coupled in parallel to the same wall outlet 52 as PLC 60. Household appliance 58 is coupled in parallel to the same wall outlet 54 as PLC 62.
The noise on power lines can be quite high due to electrical interference emanating from the very devices being powered. It is well-known that most electronic and electrical products (e.g., household appliances such as household appliances 56 and 58) generate significant electrical noise which is eventually coupled back across the power lines. Signal attenuation on the power line results primarily from the impedance of the power wiring in conjunction with the impedance of various power line loads. Specifically, in the case of house 49, the attenuation from PLC 60 to PLC 62 results from a concatenation of voltage dividers formed by 1) the output impedance of PLC 60 and the parallel impedance of appliance 56; 2) the series impedance in lines 53a and the parallel equivalent impedance of lines 53c including their associated loads; and 3) the series impedance of lines 53b and the parallel impedance of appliance 58. If, continuing the example, outlets 52 and 54 are connected to different power line phases, then there will be additional attenuation due to imperfect signal coupling between power line phases.
Practitioners in the communications art have long recognized that the combination of signal attenuation plus high noise levels poses an especially difficult problem. It is this combination which challenges a power line carrier system in its ability to provide reliable communications.
In the prior art, spread spectrum communications techniques have been utilized in noisy environments, including power lines. Spread spectrum involves transmitting communications signals over a frequency spectrum that is purposely made broad with respect to the information bandwidth. The signals are subsequently despread and decoded at the receiving end. In this manner, spread spectrum receivers have the highly desirable ability to enhance the expected signal while suppressing the effects of all other inputs. One of the drawbacks, however, in using spread spectrum communications is that it often requires elaborate, complex and expensive circuitry. Certain types of spread spectrum systems also suffer from prohibitively long acquisition, synchronization and decoding times.
Many attempts have been made to overcome these shortcomings. One attempt is shown in U.S. Pat. No. 4,979,183. What is needed is a simple spread spectrum communications system which is more reliable for transmitting and receiving data in a network of intelligent cells Which provide sensing, control and communications.
A digital implementation would provide advantages over prior art analog approaches in that certain storage functions are easily implemented. Furthermore, digital implementations are less sensitive to circuit layout. Digital implementations can also be brought to market more rapidly. Finally, digital implementations allow straight forward cost reductions as IC process technology advances.
As will be shown, the present invention provides a simple, low-cost means of providing reliable spread spectrum communication. The present invention employs spread spectrum communication techniques which have a high immunity from interfering signals. Furthermore, the present invention provides a system having an implementation which utilizes an all digital core (only analog input and output interfaces are employed) for providing spread spectrum communications.